In circuits that can be programmed using a resistor, various operating states or various operating parameters can be set by a user using an external resistor. In this context, the operating states or operating parameters are set using an external resistor which the user can connect to an output terminal of the circuit. For each of the different operating states or operating parameters, one or more resistors with a prescribed resistance value needs to be connected.
The circuit ascertains the resistance value of the connected resistor and, on the basis of the ascertained resistance value, generates a control signal which sets the operating state or operating parameter. To ascertain the resistance value, a reference voltage is applied across the external resistor, and the output current flowing through the external resistor via the output terminal is ascertained. In this case, the number of different values which this output current can assume corresponds to the number of different operating states or operating parameters which can be set using the external resistor.
Besides a reference voltage source generating the reference voltage applied across the external resistor, such circuits usually also require exact reference currents. A reference current of this kind can be generated, in principle, by applying an exact reference voltage to a nonreactive resistor. In this connection, a prerequisite for exact generation of a reference current is that a reference voltage is generated which is subject only to small fluctuations, and that the resistor has exactly the resistance value which is required in order to generate the desired reference current taking into account the reference voltage. An exact reference voltage can be generated in integrated circuits using a bandgap circuit, for example. However, producing a nonreactive resistor whose resistance value can be set exactly and is subject only to small production-related fluctuations is barely possible in integrated circuits, or is possibly only with a very high level of complexity.
A reference current for integrated circuits is therefore usually generated using external resistors which can be produced with comparatively small production-related fluctuations, usually less than 1%. However, connecting an external resistor of this kind in order to generate an internal reference current requires an additional connection terminal, whose implementation is associated with additional space requirement and with additional costs.